Foams of thermoplastic resins, such as, for example, polystyrene and polyethylene, are useful industrial products because of their excellent heat insulating, cushioning and other properties. These foams have found acceptance over the years in such applications as thermal insulation and cushioning and as raw materials for the fabrication of various shaped articles.
The preparation of thermoplastic foams by extruding a heat-plastified mixture of thermoplastic resin and a blowing agent is well known in the art and is described in U.S. Pat. Nos. 2,740,157; 3,067,147; 3,413,387; 3,413,388; 3,431,163; 3,431,164; 3,954,929; and 3,966,381, and Canadian Patent No. 451,864, as well as in other literature pertaining to the art.
U.S. Pat. No. 2,450,436 discloses a method for the preparation of cellular thermoplastic products in which a solid thermoplastic resin, e.g., polystyrene, and a normally gaseous agent such methyl chloride, methyl ether, propylene or butylene are held in a closed vessel under pressure at a temperature below the critical temperature of the normally gaseous agent until a homogeneous mobile gel is obtained. Thereafter, an outlet is opened to permit flow of the gel from the vessel. During flow of the mobile gel from the pressurized vessel into a zone of lower pressure, the resin is swollen by vaporization and expansion of the dissolved volatile substance to form a stable cellular product consisting for the most part of individual thin-walled closed cells.
U.S. Pat. No. 2,515,250 describes a method of forming a mixture of a normally gaseous agent and a thermoplastic resin under pressure, and storing the mixture by feeding it into a pressurized storage vessel in which it is maintained at a desired temperature until a homogeneous mobile gel or solution is obtained prior to extrusion and expansion of the resin.
U.S. Pat. No. 3,067,147 discloses a method for the preparation of a cellular mass from thermoplastic resins by incorporating in the material to be foamed a gas or volatile organic liquid with a thermoplastic resin, the latter then being heated to a temperature at which it becomes plastic, whereby vapors of gas or volatile liquid expand the softened resin to form a cellular mass.
U.S. Pat. No. 2,387,730 teaches the making of cellular polyethylene by impregnating a molten polymer with a gas which is soluble therein under pressure, then partially releasing the pressure while maintaining the temperature, to expand the polymer, and then cooling the expanded polymer.
U.S. Pat. No. 2,174,779 to Delorme describes apparatus for extruding various colored plastics in which a predetermined design is obtainable by feeding different colors of plastic material through an extruder. Foamed materials are not disclosed.
The patent to Wiley, U.S. Pat. No. 3,769,380, also describes a method for extruding synthetic thermoplastic sheet material having a variegated colored pattern. The method describes means for obtaining a variegated pattern, advantageously a wood-grain pattern, in the formation of non-foamed multicolored plastics.
Randall, U.S. Pat. No. 4,011,292, describes a method for producing sheet material with a multicolor striated pattern and Wissinger, U.S. Pat. No. 4,128,386, describes an apparatus for the extrusion of patterned sheets or panels of thermoplastic synthetic resin compositions. Neither of these two patents describes a method for making multicolored foam materials.
In order to obtain multicolored or multidensity foams, it has previously been necessary to laminate together sections of foams of different colors and/or densities. This additional procedure adds labor, and therefore costs, to the product and, moreover, results in a non-continuous interface between the various colors and/or types of foam with resultant poor aesthetic qualities and structural weaknesses if proper bonding does not occur.